System and method of detecting a blocked aperture in letter or flat mail image sensor

ABSTRACT

Embodiments include systems and methods of detecting a blocked aperture in an image device. In certain embodiments, the system and method is used in mail processing of letters and flats. In certain embodiments, the image sensor captures an image of the front of an item. If the aperture of the image sensor is obstructed, a void will appear on the image of the item. The system can detect the void and increment an alarm count until a pre-defined threshold is reached wherein the system signals an alarm so that the blockage can be removed and the affected items can be re-introduced for correct processing. In another embodiment, images obtained when the aperture is expected to be clear of any items.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and is a continuation of U.S.application Ser. No. 12/569,718, filed Sep. 29, 2009, the entirecontents of which are hereby incorporated by reference. Any and allapplications for which a foreign or domestic priority claim isidentified in the Application Data Sheet as filed with the presentapplication are hereby incorporated by reference under 37 CFR 1.57.

BACKGROUND OF THE INVENTION

Field of the Invention

The embodiments presented herein relate to detecting a blockage or avoid on the aperture of an image sensor.

This application claims priority to and is a continuation of U.S.application Ser. No. 13/160,421, filed Jun. 14, 2011, which applicationclaims priority under 35 U.S.C. §119 to Korean Patent Application No.10-2010-0107826, filed on Nov. 1, 2010, the entire contents of which arehereby incorporated by reference. Any and all applications for which aforeign or domestic priority claim is identified in the Application DataSheet as filed with the present application are hereby incorporated byreference under 37 CFR 1.57.

Description of the Related Technology

Image sensors are components mounted on the belt transport plate ofsorting machines. In one embodiment, the sorting machines are configuredto receive mail items. In one non-limiting embodiment, these imagesensors are used in postal operations to capture images of the front orback side of the mail and to process the images in order to detect anddecode the information printed on the mail piece for mail processingautomation. In certain embodiments, these images are barcodes. OpticalCharacter Reading is one additional function that can be implemented onsuch image sensors.

With letter and flat mail sorters, the mail travels in front of theimage sensor and in close proximity with the face plate of the imagesensor head and with the sapphire window (aperture) of the sensor. Theimage sensor scans the item one column of the item at a time from oneend or edge of the item to the other.

One of the most common causes of failure in such devices is foreignmatter that becomes attached to the outer side of the aperture window.Such foreign matter prevents the light of the image sensor light sourcefrom reaching the target mail piece and blocks the light reflected bythe target mail piece from reaching the image sensor. Most often,foreign matter can be dust or debris in various amounts, glue mixed withdust or pieces of paper of different sizes including paper labels withadhesive backing, frequently used in mail preparation and processing. Ifthe image sensor aperture is blocked and the image cannot be scanned ina way that the rest of the system can continue processing the item,there is generally no way to timely tell that there is a problem withimage sensor aperture.

Typically, this kind of blockage may occur when a mailing label orbarcode label or some other type of label or sticker which is affixed tothe item dislodges, gets separated from the envelope and sticks to theaperture, partially or totally blocking the aperture. When this happens,the image sensor does not get a complete image of the item. However,because the system is moving at such a high speed and the volume ofitems passed in front of the aperture is so large, hundreds or eventhousands of pieces of mail therefore pass through the system before itis realized that there is a problem. Further, all of the items thatpassed in front of the image sensor likely need to be reprocessed due tothe fact that the image sensor that is reading the information has ablocked aperture.

SUMMARY OF CERTAIN INVENTIVE ASPECTS

The system, method, and devices of the invention each have severalaspects, no single one of which is solely responsible for its desirableattributes. Without limiting the scope of this invention as expressed bythe claims which follow, its more prominent features will now bediscussed briefly. After considering this discussion, and particularlyafter reading the section entitled “Detailed Description of CertainEmbodiments” one will understand how the features of this inventionprovide advantages over the existing technology.

One embodiment includes system for detecting blockage of an apertureassociated with an image sensor as a plurality of items are sequentiallyadvanced proximal to the aperture. The system includes an image sensorconfigured to obtain an image of a first item having been advanced to aposition proximal to an aperture through which the image sensor imagesat least portion of the item. The system further includes a memoryconfigured to store data associated with at least one previouslyobtained image of an item advanced proximal to the aperture. The systemfurther includes at least one processor configured to: identifyinformation indicative of a void in the image of the first item, comparethe identified information indicative of the void in the image of thesecond item with stored information indicative of a void in a previouslyobtained image of the first item, and provide an indicator of a blockageof the aperture of the image sensor based on the comparison

One embodiment includes a system for detecting blockage of an apertureassociated with an imager as a plurality of items are sequentiallyadvanced proximal the aperture. The system includes an imager configuredto obtain an image of an item having been advanced to a positionproximal an aperture through which the imager images at least a portionof the item, a memory configured to store data associated with at leastone reference image obtained at a first time when no item was proximalto the aperture, and at least one processor configured to: instruct theimager to obtain a second image at a second time when no item isexpected to be advanced proximal to the aperture, compare dataassociated with the second image with the stored data associated withthe at least one reference image, and provide an indicator of a blockageof the aperture of the imager based on the comparison.

Other embodiments include methods associated with such embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of mail sorter system equipped with an imagerand image processing system according to one embodiment.

FIG. 2 is a block diagram illustrating in further detail a portion ofthe system illustrated in FIG. 1.

FIG. 3 shows a flowchart illustrating a method of detecting a blockedaperture in an image processing system such as illustrated in FIG. 1.

FIG. 4 shows a flowchart illustrating another method of detecting ablocked aperture in an image processing system such as illustrated inFIG. 1.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

The following detailed description is directed to certain specificembodiments of the development. However, the development can be embodiedin a multitude of different ways as defined and covered by the claims.In this description, reference is made to the drawings wherein likeparts are designated with like numerals throughout.

Embodiments include systems and methods of detecting blockage of anaperture through which an imager captures of images of items being movedpast the imager. For example, in one embodiment, a processor searchesfor horizontal patterns (bands or voids) in captured images that stretchover the whole length of the image, and which may extend beyond theedges of the item. These bands will be very stable on the vertical axisbetween images and their stability will allow the image sensor softwareto distinguish them from any other horizontal bands that are part of theitem graphic design. The latter will move up and down as the mail itemstravel at changing heights above the base of the sorter transport. Whensuch patterns are found on a predetermined number consecutive mail itemsin sequence, a “blocked aperture” signal is generated.

In another embodiment, a reference image is taken during the imagesensor calibration or at a moment when there is no item in front of theimage sensor aperture and the aperture is clean and free of debris orany other foreign matter. To capture the image, a light source isstrobed in a time interval when no mail is blocking the aperture. Thisreference image is stored for comparison. Periodically, the imagesensor, synchronized with the strobe signal, captures an image when noitem is anticipated to passing by the aperture. The processor comparesthis image with the reference image. When the differences between thetwo images are less than a specified threshold, the processor softwaregenerates a “blocked aperture” signal. The threshold may be defined ingray scale or color levels and an optimal value is determined throughcalibration with a particular apparatus.

It is to be recognized that in certain embodiments, the two methodsdescribed above can be combined to increase the accuracy of the blockagedetection process.

FIG. 1 is a block diagram of mail sorter 122 equipped with an imager andprocessing system according to one embodiment. In FIG. 1, an item 120 tobe processed is received by the item sorter 122 via an item feeder 150.The item 120 is provided to an image capture system 200, which comprisesan imager 205, processor 250, software 260 and a memory 280. Forillustrative purposes of this embodiment, the imager 205 includes ascanner comprising an image sensor, a light source, and image formingoptics. In the illustrated embodiment, the image capture system 200 isconfigured to read item delivery and other related information from amail piece. The imager 205 is positioned proximal a slot or transparentopening in a faceplate 150 that provides an aperture 240 through whichthe imager 205 may image items 120 as the pass along the faceplate 150.The aperture 240 may include a window of, for example, a hardenedtransparent material such as a sapphire window. In certain embodiments,the item 120 to be processed is a mail piece. In one non-limitingexample, a mail piece, for instance, may be a large manila envelope, amagazine, or a letter. In other embodiments, the item 120 can be apackage or box. It is to be recognized that in some embodiments the itemmay be a product on an assembly line or a part on a production line oran item in a warehouse system. After being advanced past the imager 205,the item 120 proceeds to further components of the sorter 122, e.g.,further gates, diverters, and bins illustrated generally as a mailprocessing block 300.

As the mail piece is advanced past the aperture 240, the imager 205captures an image of the item 120 by scanning an image of the item 120,one column of pixels at a time until the entire image is captured, whilethe item is passing in front optics and sensor of the imager 205. Incertain embodiments, the image is the entire surface of the item 120being scanned.

In one embodiment, the feeder system 150 for advancing the item 120 pastthe imager comprises a conveyer belt. In other embodiments, it cancomprise any suitable machine that advances the item to the imager 205and positions the item 120 for imaging by the imager 205. In theillustrated embodiment, the feeder system 150 receives the item 120 in aproper, e.g., vertical, on its edge facing the imager 205, orientationfor scanning and processing from other item processing equipment.Generally, the item 120 does not stop moving while advancing past theimager 205. Rather, the imager 205 scans an image of the item as it isadvanced past the aperture.

After the image of the item 120 has been captured, the processor 250 ofthe image capture system 200 processes the image according to a method,for example, implemented by executing a computer program 260 to detectif a void indicative of blockage of the aperture 240 is detected in thecaptured image of the surface of the item 120. In one embodiment, thememory 280 stores the computer program instructions. In certainembodiments, the memory 280 stores information about the image of theitem captured by the imager 200.

In operation, as the item 120 is passing the camera aperture, materialssuch as mailing labels can be dislodged and adhere or attach to theaperture 240, resulting in a partially or totally blocked aperture. Theblockage of the aperture 240 may be caused, for example, by a mailinglabel or other label on the item 120 being scanned adhering to, oradhering proximal to, the aperture 240. A blockage may also be caused bydirt, dust or other debris blocking the aperture 240. Generally, ablockage may be caused by anything that results in the aperture 240being partially or fully blocked. When part or all of the lens oraperture is blocked, the blockage may appear as a horizontal band thatdefines a void in the image being captured. For example, a horizontalband that repeatedly appear in consecutive image captures in the sameapproximate location can be indicative of a blockage because asubstantially identical band would not be likely to appear in a numberof successive images unless resulting from some material other than theitems being imaged. Hence, if a void is detected for one item 120, thecoordinates of the void are stored in memory 280. In one embodiment, thecoordinates of the void are the location of the void relative to thecamera. If the same coordinates, within a certain predefined range, areidentified as being a void indicative of a possible blockage, then analarm count is incremented upon processing each such item. At the pointwhen the alarm count reaches a predefined threshold amount, the system100 outputs a message indicating the blockage. In some embodiments, theoutput message will result in the machine stopping until the aperture ofthe camera lens is cleared. If the item image appears to be error free,the item 120 advances to further normal processing 300. The thresholdvalue can be set or changed by the user or vendor at any time. Thethreshold value refers to the maximum number of items 120 with adetected void that the system 100 will allow to process without sendingan alarm or notification.

FIG. 2 depicts one embodiment of an image capture system 200 as used inthe sorter 122 in more detail. It is to be appreciated that the figureis not to scale. Moreover, sorter/feeder components are also not shownto simplify the view. An item 120 is advanced to the image capturesystem 200 via a transport system (not shown) of the sorter 122. Theillustrated portion of the imaging capturing system 200 includes theface plate 215 in which the slot 210 is provided and which defines theposition of the aperture 240. The imager 205 is positioned proximal theaperture 240. As discussed above, the item 120 advances proximal to andpast the aperture 240 to allow the imager 205 to capture a digital imageof the surface of item 120 by scanning one column of pixels at a time insuccession until an image of the entire surface of the item 120 has beencaptured. As also discussed above, when the items 120 advance passed theaperture 240, stickers, labels and the like can sometimes come dislodgedfrom the item 120 and subsequently adhere to the aperture 240 andpotentially partially or wholly obstructing the aperture 240 of theimager 205. Without a way of detecting a blocked aperture, items areunnecessarily reprocessed and the blockage may not be noticed untildetected by overloading of subsequent reject processing systems.

FIG. 3 is a flow diagram illustrating one embodiment of a method 300 ofdetecting a blockage of the aperture 240. Beginning at block 310, theitem feeder 150 receives an item 120 and advances the item 120 to theimage capture system 200.

Moving to block 315, the item 120 passes the imager 205 and is scannedfor image processing. The item 120 advances past the aperture 240 andthe imager 205 captures an image of the item 120 at a high rate ofspeed.

Moving to a decision state block 320, the processor 250 determines if avoid is detected across substantially the entire captured image. In oneembodiment, the void appears as a horizontal band in the scan in theplace where part of the item image would be. The determination of a voidmay be based on a minimum threshold of thickness the coordinates of thevoid. This threshold as to what constitutes a void can be set by theeach vendor of the software 260 to be tailored to different needs ofvarious systems, customers or users.

If there is no void detected then the item 120 continues to processnormally at block 390. In one embodiment, processing normally is thecontinued routing of the mail piece. In other embodiments, processingnormally is whatever next steps occur when the item exits the imagesensor image sensor.

If a void was detected in decision block 320, the process 300 moves toblock 325, where the distance is calculated from the bottom of the scanto the beginning of the void. The distance is calculated by the imageanalyzing software within the system. In one embodiment, the processor250 can be configured to analyze the image using software that can beoff-the-shelf. In another embodiment, the image analyzing functionalitycan be part of the software system 260.

Moving to block 330, the thickness of the void is then calculated. Thethickness may also be referred to as the height. In one embodiment, thethickness is calculated by determining the distance from the start ofthe void until the start of the image of the item 120 or else a boundaryof the item 120. In other embodiments, other ways of measuring thethickness are used.

The process 300 then moves to decision state bock 335. Here, theprocessor 250 determines if the item 120 currently scanned is the firstitem 120 for which the void has been detected. For example, theprocessor 250 checks the memory 280 to determine whether or not thereare stored coordinates in memory from the previous image of the previousitem 120. If no, indicating that there is not a set of coordinatesindicative of a previous void stored, then the processor 250 determinesthat the current item 250 is the first item to have its scanned imageblocked by the void, and moving to a block 350, the increment counteralarm is incremented by 1. Hence, the alarm count is configured tomaintain count of how many items 120 that have been processed have asubstantially identical void so that when the count reaches apre-defined threshold, corrective action may be taken.

Referring again to block 335, if a “yes” has been returned indicatingthat a set of coordinates from the previous image is stored, and themethod 300 advances to a block 340 at which the processor 250 comparesthe coordinates of the current detected void to the coordinates storedin the memory 280 regarding the previously detected void(s).

Next at a decision state 345, the processor 250 determines whether thestored coordinates and the current coordinates are the same. If thecoordinates are not the same, then the method 300 advances to block 370.At this state the new coordinates are stored in the memory 380 and thealarm count is set to 1. Moving from the block 370, the method 300advances to a block 390 at which normal image processing proceeds. If atthe block 345, the coordinates are not the same, it is usuallyindicative of a situation when the previous void is an anomaly. In thesecases, the sorter 122 does not need to be stopped or delayed as there islikely no ongoing issue, but rather an issue for a small number of itemsthat self corrects.

Hence, next at the decision state 345, if it is determined that thecoordinates are the same, then the method 300 advances to the block 350at which the processor 250 increments the alarm count (e.g., adds one tothe stored alarm count).

The process 300 next moves to decision state block 355, and theprocessor 250 compares the value of the alarm count with the value ofthe previously defined threshold to see if it exceeds the threshold thathas been set. In one embodiment, the threshold is five. In otherembodiments the threshold is determined by the vendor or the user of thesystem. The threshold value can be changed within the software by thevendor or user without having to perform an upgrade or reprogramming ofthe software.

If the alarm count exceeds the threshold, the process 300 advances toblock 360 at which the in-feed line (which feeds the items 120 past theimager 250) is stopped. The sorter 122 is then cleared. In someembodiments, this clearing can be done mechanically. In otherembodiments, the system alerts a technician or other person who isinformed that the image sensor aperture 240 is to be cleared. After thevoid is cleared, the alarm count is set to zero and the sorter 122 canbe restarted.

Different methods can be employed to send an alert regarding a blockedaperture. When the blocked aperture detection mechanism indicates a“blocked” condition, the processor generates a signal that is output viaan audio alarm, a warning lamp, and/or video device to inform theequipment operator that corrective action is to be taken. A signal canalso be sent to other control portions of the sorter 122 to stop themail transport so that the blocked aperture can be cleared before mailprocessing can be resumed.

In one embodiment, a software message signal may be used. In theseembodiments, the signal is a flag set in a digital message sent by theprocessor 250 to the mail sorter host computer and displayed on the hostcomputer monitor screen.

In certain embodiments, the signaling methods described above can becombined based on the overall equipment design.

Referring again, to block 355, if the alarm count does not exceed thethreshold, then the process 300 advances to block 390 and item 120 isallowed to process normally.

In certain embodiments, the methods described above can be used todetect a barcode reader blocked aperture condition on letter and flatmail sorters in the USPS or any other item processing facility outsideUSPS.

FIG. 4 shows a flowchart illustrating a method 400 of detecting ablocked aperture in the sorter 122. The method begins at a block 402 inwhich the processor 250 instructs the imager 205 to obtain at least onereference image at a first time when no item is proximal to the aperture240. The reference image is stored to the memory 280. In one embodiment,the first time is during a specified calibration period, or at a timewhen the sorter 122 is started, e.g., at the start of item processing orsubsequent to a prior blockage being cleared. Next at a block 404, theprocessor 250 instructs the imager 205 to obtain a second image at asecond time when no item is expected to be advanced proximal to theaperture 240. The second time is generally subsequent to the first timeand can be periodically during operation of the sorter 122.

Moving to a block 406, the processor 250 compares data associated withthe second image with the stored data associated with the at least onereference image. The comparison can be performed, for example, on apixel by pixel basis of all or part of the image. Proceeding to a block408, the processor 250 generates an indicator of a blockage of theaperture 240 of the imager 205 based on the comparison. As describedabove, the indicator can be output as a signal via an audible alarm, avisual alarm such as on an indicator lamp or electronic display screen.In one embodiment, the sorter 122 may automatically stop processing inresponse to the alarm to allow any blockage to be cleared.

It is to be recognized that depending on the embodiment, certain acts orevents of any of the methods described herein can be performed in adifferent sequence, may be added, merged, or left out all together(e.g., not all described acts or events are necessary for the practiceof the method). Moreover, in certain embodiments, acts or events may beperformed concurrently, e.g., through multi-threaded processing,interrupt processing, or multiple processors, rather than sequentially.

Those of skill will recognize that the various illustrative logicalblocks, modules, circuits, and algorithm steps described in connectionwith the embodiments disclosed herein may be implemented as electronichardware, computer software, or combinations of both. To clearlyillustrate this interchangeability of hardware and software, variousillustrative components, blocks, modules, circuits, and steps have beendescribed above generally in terms of their functionality. Whether suchfunctionality is implemented as hardware or software depends upon theparticular application and design constraints imposed on the overallsystem. Skilled artisans may implement the described functionality invarying ways for each particular application, but such implementationdecisions should not be interpreted as causing a departure from thescope of the present invention.

The various illustrative logical blocks, modules, and circuits describedin connection with the embodiments disclosed herein may be implementedor performed with a general purpose processor, a digital signalprocessor (DSP), an application specific integrated circuit (ASIC), afield programmable gate array (FPGA) or other programmable logic device,discrete gate or transistor logic, discrete hardware components, or anycombination thereof designed to perform the functions described herein.A general purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with theembodiments disclosed herein may be embodied directly in hardware, in asoftware module executed by a processor, or in a combination of the two.A software module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM, or any other form of storage medium known in the art. Anexemplary storage medium is coupled to the processor such the processorcan read information from, and write information to, the storage medium.In the alternative, the storage medium may be integral to the processor.The processor and the storage medium may reside in an ASIC. The ASIC mayreside in a user terminal. In the alternative, the processor and thestorage medium may reside as discrete components in a user terminal.

While the above detailed description has shown, described, and pointedout novel features of the invention as applied to various embodiments,it will be understood that various omissions, substitutions, and changesin the form and details of the device or process illustrated may be madeby those skilled in the art without departing from the spirit of theinvention. As will be recognized, the present invention may be embodiedwithin a form that does not provide all of the features and benefits setforth herein, as some features may be used or practiced separately fromothers. The scope of the invention is indicated by the appended claimsrather than by the foregoing description. All changes which come withinthe meaning and range of equivalency of the claims are to be embracedwithin their scope.

What is claimed is:
 1. A system for detecting a blockage of an apertureassociated with an image sensor as a plurality of items are sequentiallyadvanced proximal to the aperture, the system comprising: an imagesensor configured to obtain an image of at least a portion of a firstitem having been advanced to a position proximal to an aperture throughwhich the image sensor images the at least a portion of the first item,and to obtain an image of a second item, the second item having beenadvanced to a position to the aperture; a memory configured to storedata associated with the image of the first item and the image of thesecond item; and at least one processor configured to: identify a voidin the image of the first item; determine a size and a location of thevoid in the image of the first item; identify a void in the image of thesecond item; determine a size and a location of the void in the image ofthe second item; compare the size and the location of the void in theimage of the first item with the size and the location of the void inthe image of the second item; and provide an indicator of a blockage ofthe aperture of the image sensor based on the comparison.
 2. The systemof claim 1, wherein the item is a mail piece.
 3. The system of claim 1,wherein the image sensor is a camera.
 4. The system of claim 1, whereinthe image sensor is a scanner.
 5. The system of claim 1, wherein theprocessor is further configured to: increment a value when at least oneparameter respectively associated with the void in the image of thefirst item and associated with the void in the second item matcheswithin a specified threshold; and based on the incremented value, outputthe indicator of a blockage.
 6. The system of claim 1, wherein theprocessor is configured to identify the size and location of the voidsin the images of the first and second items by: calculating a distancefrom the bottom of the image of the first item to the beginning of thevoid in the image of the first item: and calculating a thickness of thevoid in the image of the first item; calculating a distance from thebottom of the image of the second item to the beginning of the void inthe image of the second item: and calculating a thickness of the void inthe image of the second item; and wherein the size and the location ofthe void in the image of the first item comprises the distance from thebottom of the image of the first item and the thickness of the void inthe image of the first item and wherein the size and location of thevoid in the image of the second item comprises the distance from thebottom of the image of the second item and the thickness of the void inthe image of the second item.
 7. A system for detecting a blockage of anaperture associated with an imager as a plurality of items aresequentially advanced proximal the aperture; the system comprising: animager configured to obtain an image of an item having been advanced toa position proximal to an aperture through which the imager images atleast a portion of the item; a memory configured to store dataassociated with at least one reference image obtained at a first timewhen no item was proximal to the aperture; and at least one processorconfigured to: instruct the imager to obtain a second image at a timewhen no item is expected to be advanced proximal to the aperture;identify a void in the at least one reference image; determine a sizeand a location of the void in the at least one reference image; identifya void in the second image; determine a size and a location of the voidin the second image; compare the size and the location of the void inthe second image with the size and the location of the void in the atleast one reference image; and provide an indicator of a blockage of theaperture of the imager based on the comparison.
 8. The system of claim7, wherein instructing the image sensor to obtain a second imagecomprises strobing a light source associated with the imager.
 9. Thesystem of claim 7, wherein the item is a mail piece.
 10. The system ofclaim 7, wherein the imager is a camera.
 11. The system of claim 7,wherein the imager is an image sensor.
 12. The system of claim 7,wherein the processor is further configured to: increment a value whenat least one parameter respectively associated with the void in theimage of the at least one reference image and associated with the voidin the second image matches within a specified threshold; and based onthe incremented value, output the indicator of a blockage.
 13. A methodof detecting a blockage of an aperture associated with an image sensoras a plurality of items are sequentially advanced proximal the aperture,the method comprising: advancing an item past the image sensor;obtaining an image of the item via the image sensor; storing informationassociated with the image data to a memory; detecting a void in theimage data; determining a size and a location of the void in the imagedata; determining a size and a location of a previously stored void;comparing the size and the location of the void in the image data to thesize and the location of the previously stored void; incrementing analarm count indicating a number of consecutive voids detected; comparingthe alarm count to a threshold value; and generating an alert based onthe comparing.
 14. The method of claim 13, wherein obtaining an imagecomprises scanning vertical rows of pixels representing the item. 15.The method of claim 13, wherein comparing the size and the location ofthe void in the image data to the size and the location of thepreviously stored void comprises comparing coordinates of the voidsrelative to the image dimensions.
 16. The method of claim 13, whereincomparing the size and the location of the void in the image data to thesize and the location of the previously stored void comprises comparingcoordinates of horizontal thicknesses of the voids relative to the imagedimensions.
 17. The method of claim 15, further comprising resetting thealarm count to one when the coordinates compared are not within apredefined range of each other.
 18. The method of claim 13, furthercomprising determining if the alarm count has reached a threshold numberof images with voids.
 19. The system of claim 1, wherein the processoris further configured to initiate a mechanical cleaning of the blockedaperture.
 20. The system of claim 7, wherein the processor is furtherconfigured to initiate a mechanical cleaning of the blocked aperture.21. The method of claim 13, further comprising mechanically cleaning theblocked aperture.